Add kube-apiserver, kube-controller-manager, kube-scheduler and etcd to glossary. (#6600)

* Move master component definitions to glossary

* review comments

* Update controller.yaml

Making a trivial change to un-stuck Travis

* Update controller.yaml

Removing trivial change

_data/glossary/etcd.yaml

@@ -0,0 +1,11 @@
+id: etcd
+name: etcd
+full-link: /docs/tasks/administer-cluster/configure-upgrade-etcd/
+tags:
+- architecture
+- storage
+short-description: >
+  Consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.
+long-description: >
+  Always have a backup plan for etcd's data for your Kubernetes cluster.
+  For in-depth information on etcd, see [etcd documentation](https://github.com/coreos/etcd/blob/master/Documentation/docs.md).